The present invention is in a process of purifying an aqueous effluent which results from a scrubbing of the exhaust air from a paint, lacquer or varnish spray booth.
In a paint, lacquer or varnish spray booth, as much as 50% of the paint, lacquer or varnish becomes suspended in the air which is contained in or supplied to the spray booth. As a result, the exhaust air from a spray booth contains solvent droplets and vapors as well as particles of coloring matter. The solvents contained in the exhaust air from the spray booths are separated by an adsorbent and are subsequently recovered by desorption. The particles of coloring matter must be separated from the exhaust air by a scrubbing treatment prior to the solvent adsorption because those particles would deposit on the adsorbent and would interfere with the solvent separation.
It is already known that the exhaust air from paint, lacquer or varnish spray booths can be exhausted and scrubbed with water in a scrubber. The scrubbing water takes up parts of the solvent and the particles of coloring matter, some of which are very sticky, and for this reason must be purified. This purification can be accomplished by a treatment with a flocculating agent, such as bentonite, so that the particles of coloring matter and fractions of the solvent become bonded in the sludge. The sludge is separated by sedimentation or filtration and after a partial dewatering is placed on a dump. The water is returned to the scrubbing cycle.
That known process has particularly the disadvantage that large quantities of sludge must be dumped and the particles of coloring matter separated from the aqueous effluent cannot be returned to the coating process. For instance, in a plant for applying lacquer or varnish in 30 coating trains for coating 12,240 car bodies in 24 hours, 180,000 m.sup.3 of aqueous effluent containing 12,200 kg particles of coloring material will become available during that period. Flocculating agent in an amount of 288,000 kg will be required in 24 hours for the separation of the particles of coloring material.
For this reason it is an object of the invention to provide a process for the purification of the aqueous effluent from treatment of the exhaust of paint, lacquer or varnish spray booths, in which sewage sludge that must be dumped will not be formed and which permits recycling of the particles of coloring material contained in the aqueous effluent.